Bruce Walton, collisions analyst for Thames Valley Police Safer Roads Partnership, said, "Basically motorways early on a Friday morning are a bad place to be, especially in November."

Mr Walton said there were 6% more accidents on Fridays caused by tiredness or driver inattention than the daily average, according to figures for some 6,500 accidents in the force area over the three years from November 1999 to December 2002.

There were 5.5% more crashes involving fatigue in November than the monthly average, and 28% more than the average per hour between 3am to 4am. Figures were higher than average for all times from 3am to 7am.

Driver fatigue or inattention accounted for 40% of deaths and serious injuries on motorways, but just 20% on other roads in the force's three counties of Oxfordshire, Berkshire and Buckinghamshire.

Mr Walton said, "Friday is the end of the working week so that is a factor, while with 3am to 7am you have people who have been driving all night who haven't stopped.

"The reason why November is worse is an interesting one, but we often relate that to the clocks going back."

Figures from the last five years showed that there were 11% more injury accidents during rush hours in the week after the clocks went back, he said.

Inspector Malcolm Collis, head of Thames Valley Police's roads policing specialist units, warned motorists they could end up in court for falling asleep.

"Although there is no specific offence of driving while tired, you could still be prosecuted for dangerous or careless driving if you fall asleep at the wheel, that is, if you survive," he said.

"Fatigue-related crashes are often more serious because if the driver falls asleep he or she will not brake before impact, making the speed of impact far greater.

"It is impossible to force yourself to stay awake. You must do something about it."

Mr Collis advised drivers:

Do not drive for more than one hour between midnight and 6am.

Take a 15-minute break every two hours for every journey.

During breaks have two cups of caffeinated drink and take a 10 to 15 minute nap (but no longer).

Thames Valley Police covers the biggest area of motorway in Britain, stretching 196 miles including parts of the M4, M40, M25, and M1, among the busiest in Europe.